1. Field of the Invention
This invention relates to primer compositions. MOre particularly, this invention relates to primer compositions capable of achieving cohesive bonding between organosiloxane elastomers and organic polymers. The primers are particularly useful for preparing optically transparent laminates comprising layers of theroplastic organic polymers bonded by a cured organosiloxane elastomer. These laminates can be used as windows and windshields in vehicles and aircraft.
2. Description of the Prior Art
Organosilicon compounds useful for bonding organosiloxane elastomers to various substrates are disclosed in U.S. Pat. No. 4,719,262 issued to Edwin Plueddemann on January 12, 1988. The organosilicon compound is a silane or bissilylalkane containing at least two silicon-bonded alkoxy groups per molecule in addition to at least one substituent that is bonded to silicon through oxygen and contains at least four carbon atoms, two of which form a terminal group of the formula CH.sub.2 .dbd.CH--. A preferred group of these organosilicon compounds include reaction products of (1) an alkoxy-substituted disilylalkane such as 1,2-bis(trimethoxysilyl)ethane and (2) a monoether or diether derived from (a) an alkyl polyol such as ethylene or propylene glycol, glycerol or 1.1.1- trimethylolpropane and (b) an ethylenically unsaturated alcohol such as allyl alcohol.
If one of the substrates to be bonded is an amorphous, glassy organic polymer such as polystyrene or an ester of acrylic or methacrylic acid, in addition to at least one of the aforementioned organosilicon compounds the primer composition also contains (1) a silicone/organic copolymer derived from (a) at least one ethylenically unsaturated organic monomer such as styrene and esters of methacrylic acid or acrylic acid and (b) a silane containing three hydrolyzable groups bonded to silicon and one silicon bonded hydrocarbon or substituted hydrocarbon radical containing a terminal CH.sub.2 .dbd.CH-- radical, (2) an organohydrogensiloxane containing an average of at least three silicon bonded hydrogen atoms per molecule and (3) a solvent capable of dissolving all of the remaining ingredients in the composition.
Solvents for primer compositions comprising the mixtures of organosilicon compounds and silicon/organic copolymers disclosed in the aforementioned patent to Plueddemann include the methyl and ethyl ethers of either ethylene glycol or propylene glycol. A disadvantage of primer compositions containing these solvents is their inability to reproducibly form a thin, continuous, coherent and smooth film when applied by wiping. brushing or spraying to surfaces formed from the aforementioned amorphous, glassy organic polymers. In terms understood by those knowledgeable in surface chemistry, these compositions do not completely "wet" the surface of these polymers, resulting in unprimed areas or the presence of defects referred to as "orange peel" and "fish eyes". As a consequence of this difficulty in properly applying the primer to the substrate surface, one cannot reproducibly achieve the desired level of adhesion between the organosiloxane elastomer and the organic polymer substrates such that when force is applied to peel the elastomer layer from the substrate, separation occurs by a rupturing of the elastomer layer (cohesive failure) rather than along the interface between the elastomer and the substrate (adhesive failure).
While not wishing to be bound by any theory, the present inventors believe this inability of primer compositions containing the silicone/organic copolymers disclosed in the aforementioned Plueddemann patent to wet and cohesively bond to certain amorphous organic polymers to be due at least in part to the difference between the surface tension of the etherified polyols disclosed as solvents for these compositions and the surface energy of the polymer substrate. The present inventors observed the inability of these etherified polyols to thoroughly "wet" large areas of substrates formed from amorphous organic polymers such as polycarbonates, making it difficult to reproducibly apply to these substrates the thin. even coating of primer composition required to achieve good adhesion between the substrate and organosiloxane elastomers.
The present inventors found that while saturated aliphatic alcohols have a sufficiently low surface tension to form a coherent film when applied to amorphous organic polymers, addition of these alcohols to primer compositions containing the aforementioned silicone/organic copolymers results in formation of a solid precipitate. A variety of organic liquids were evaluated in an attempt to find those capable of dissolving the primer composition and wetting the surface of amorphous polymers such as polymethyl methacrylate and polycarbonates.